Uninterruptible power supply (UPS) systems operate in conjunction with a main power source (usually a utility) to ensure a continuous supply of energy to critical ac loads, such as medical systems, air traffic control systems, and process control instrumentation. The load power is drawn from the main power source when the main power source is operating. In emergency conditions when the main power source fails, the load power is drawn from the UPS energy source (usually a battery).
There are two main classifications of UPS systems -on-line systems and line-interactive systems. On-line systems carry the load power at all times through the UPS battery charger and inverter. In contrast, line-interactive systems normally do not carry the load power. Instead, under normal conditions they take in battery charging power from the main power supply through the inverter. In emergency conditions when the main power source fails, the line-interactive system supplies the power to the load.
A line-interactive UPS consists of three sections--a battery, an inverter, and a filter. The battery provides dc power when the main power source fails. The dc power is converted to ac power by the inverter. The filter conditions the ac power before the power is delivered to the load. The bi-directional power handling capability of the inverter is utilized to charge the battery from the main power supply under normal operating conditions.
A number of line-interactive UPS systems can be inter-connected to form a line-interactive distributed UPS system. Such a system is also known as a "secure network" since a number of paths are available to provide power to the various loads on the network. That is, different UPS units can be used to provide load power in case one of the UPS units on the network fails.
The control scheme of a distributed UPS system should not depend on signal communication between the various UPS units. The use of communication reduces the reliability of the distributed system as a whole, since the control is then critically dependent on the operation of the communication link. In addition, due to the complexity of the network, it is impractical to obtain the detailed control action for a distributed UPS network, even if the information could be conveyed to a central point.
If the voltage and frequency of the main power source degrades beyond acceptable limits, the main power source is rapidly disconnected from the secure network, resulting in an emergency condition. In an emergency condition, distributed UPS systems can, in principle, be controlled through a known power-frequency droop concept.
While there are known techniques for operating a distributed UPS system and for sharing power between various UPS units of a distributed UPS system during emergency conditions, a problem persists in defining an appropriate control mechanism to establish a desired frequency for the power signal of the distributed UPS system. This factor becomes critical in two contexts. First, assuming the use of the power-frequency droop concept to achieve power sharing between UPS units, a distributed UPS system operating under emergency conditions will experience frequency shifts as loads change on the critical network. Consequently, it is necessary to provide a mechanism to restore the power signal frequency to a desired level. Second, following re-connection of the main power source to the secure network after emergency operation, the load power must be smoothly transferred to the main source through de-centralized control.
Thus, it would be highly desirable to provide a control mechanism for a distributed UPS system that allows adjustment of the power signal frequency of the distributed UPS system to accommodate load changes and re-connection to the main power supply. Moreover, such a control mechanism should be accomplished without control signals between the UPS units of the distributed UPS system. It would also be desirable if the control scheme was operable with a random number of UPS units on the distributed system. Similarly, the location of the loads should not alter the control scheme. In addition, the control scheme should readily accommodate UPS units of different power ratings.